1 Answer

After defeating the Elite Four, this castle rises from the ground and
attaches itself to the Pokémon League building. According to a Team
Plasma Grunt, the castle was being secretly built for several years by
overworking Pokémon that were taken away from other Trainers.

Black and White 2:

>The ruins of N's Castle can accessed from a large cavern that is located within Victory Road. However, the remnants of the castle can only be accessed after the player has entered the Hall of Fame. After the player has entered the Hall of Fame, N's Zoroark stops blocking the path that leads to the ruins of N's Castle on Victory Road and lead the player there. After entering the castle, the player and N walk around the ruins that used to be N's Castle, including N's room and the castle's throne room. In the throne room, N asks the player to battle him and his ZekromB2/ReshiramW2. After the player defeats him, N reverts his dragon to its dormant state—the Dark StoneB2/Light StoneW2—and gives it to the player to summon ZekromB2/ReshiramW2 at Dragonspiral Tower to catch.

Overall:

The Castle hasn't moved at all, its main enterence has now been covered in B2W2 and is only accessible through the North of Victory Road. The Castle is where it was in Black and White, its only the way that you access it in Black and White 2 that is different.